DL380 G4 (378737-421) rack mount kit - which to choose

When searching for rack mount kits for the above ProLiant server I get 3 hits.Trouble is I bought one of them (289570-001) and the rails don't seem to fit onto the server(?)I have been told that a rack mount kit isn't necessarily applicable to a server, even it is shown when searching for one.

To me this seems a bit strange, and I'm wondering which kit to choose now?